IrO_x PH Electrode Fabrication, Response Mechanism And Its Application Study

pH is a so important parameter for daily life and industries that the fast and precise detection of pH value is of great importance. The most widely used pH sensor is the glass electrode. However, due to the intrinsic nature of the glass membrane, its drawbacks are apparent, such as high input impedance, easily broken, inadaptable to HF solutions, difficult to be miniaturized, which its their application in micro-environment and on-line detection in vivo. Therefore, many efforts have been made to develop other kinds of pH sensors, among which, metal/metal oxide electrodes were widely studied. Among the studied metal/metal oxide electrodes, IrOx electrode was suggested to be the most promising one for pH detection. This research adopted an innovative cyclic thermal oxidation and quenching method to fabricate IrOx electrodes. The property examination and characterization of different fabricated process suggested the best fabrication process. The H+ response mechanism and hydration effects on IrOx electrode were further investigated. Nafion modification of IrOx electrode made it applicable in strong reductive solutions. The H+ response process was further studied by using self-designed electrochemical cell and X-Ray absorption fine structure (XAFS) of synchrotron radiation. Besides, application of IrOx electrode in detecting the surface pH change during galvanic corrosion of Zn/steel couple were demonstrated systematically by employing a moving table.For fabricating IrOx electrodes, cycling of heating and quenching method was adopted. The examinations of Nernst response range, pH response rate, and long term stability of the different time H&Q treated IrOx electrodes, etc. suggested that the three times H&Q electrodes appeared to show the best integrated performance. Thus, additional property tests of the three-time H&Q IrOx electrode were performed, including continuous response test, destructive test, temperature coefficient test, hysteresis effect test and enlargement of pH response range test.Several detections were conducted to explore the essence of above phenomena of the electrodes undergone different cyclic times of H&Q as well as the discussion of H+ response mechanism. Morphology and composition investigations indicated a two-layer structure of the surface oxide film, i.e., dense inner layer and porous outer layer, with oxygen concentration decreases gradually from surface to iridium substrate. Electrochemical impedance spectroscopy (EIS) of the IrOx electrodes were investigated in pH buffers to analyze their pH response mechanism. Combining cross section observation and EIS results, the good performance of the three-time H&Q treated electrode is attributed to the thicker porous outer layer showing more effective hydration, larger active surface area, and smaller reaction resistance in pH detection. The H+ response mechanism of the IrOx electrode was also discussed and the response electrode/solution interface model was established. Raman spectroscopy implied good crystal quality of the iridium oxide electrode. X-Ray photoelectron spectroscopy (XPS) analyses illustrated that effective compositions of the electrode surface were Ir4+, Ir3+, and element O existed as OH’ or bound water and absorbed O2 or H2O. The composition of the electrode surface changed before and after the electrodes were applied in different pH solutions. The investigations of in-situ H+ response process was realized by using self-designed electrochemical cell and XAFS of synchrotron radiation in solutions of different pH. The in-situ/ex-situ spectra collected in solutions with different pH conditions and collected during constant potential polarization process showed the valence of Ir changed in IrOx film.Hydration is the final process in fabrication, and even an unavoidable process in maintenance and application of Ir/IrOx electrode produced by high temperature oxidation method, while this process was neglected by many authors. The impacts of hydration on Ir/IrOx electrode were examined intensively by various state-of-the-art techniques including scanning electron microscope (SEM), second ion mass spectroscopy (SIMS), X-Ray diffraction (XRD), transmission electron microscopy (TEM), XRD stress test, Raman spectra, and XPS etc. The results indicated that the grain size, interplanar spacing, single crystal quality, inner stress of the IrOx film were all changed after hydration, which may further affect the electrode potential of the electrodes. Accordingly, the reasons for electrode potential drift during long term application and the electrode potential difference of each electrode were explicated.IrOx electrodes could be used in many systems except strong reductive solutions which could cause large potential drift leading to the inaccuracy of pH measurement. XPS and ex/in-situ XAFS were used to characterize the IrOx electrode and the reason for electrode potential drift was revealed. Nafion solution was used for modification of the IrOx electrodes to make them applicable also in reductive environments. In order to achieve the best modification effect, the porous-structured IrOx electrodes were modified by being dipped into Nafion solution for different times. Surface and cross section characterizations as well as the pH detection property evaluations were conducted to identify the best modification effect by Nafion. The determination of a better modification process and the response mechanism after Nafion modification of the IrOx electrode in reducing solutions were discussed. The electrode/solution interface model of the Nafion modified electrode was also established.The application of the fabticated IrOx electrodes in detecting surface pH distribution change during Zn/steel galvanic corrosion was examined in 1mm 3.5wt.% NaCl electrolyte layer and 3.5wt.% NaCl bulk solution, respectively. Before the application of the IrOx electrode, the appropriate way of scanning the surface of the Zn/steel galvanic couple was discussed. Comparing with the case in bulk solution, the Zn/steel couple corroded faster in 1mm electrolyte layer, the surface pH of which increased to a high value after the corrosion began for 20-40min, and the low pH region existed on the zinc surface and the region adjacent to the zinc surface of the steel surface, whereas the pH value of the steel surface away from the zinc was relative high. It is demonstrated that the IrOx electrode could detected the pH distribution effectively in surface region during metal corrosion, which could help the understanding of corrosion process under different situations.